In recent years, research and development efforts have been intense on developing combinations of light sources and phosphors that will yield useful, high-performing light emitting devices, with the result that both efficient high-power light sources and efficient phosphors have been demonstrated. For example, both light emitting diode (“LED”) chips and phosphors for phosphor-converted LED (“pcLED”) devices have been demonstrated. A unique aspect of some phosphor/light source combinations (such as pcLEDs) is that the phosphors are in contact with the light source (such as a LED chip), and the light sources operate at high temperatures. For example, typical junction temperatures of high power LEDs are in the range of 80° C.-150° C. At these temperatures, the crystal of the phosphor is at a high vibrationally excited state, causing the LED excitation energy to be directed to heat emission through lattice relaxation rather than to the desired luminescence emission. Moreover, these lattice relaxations produce further heating, thereby further reducing the luminescence emission. This is a vicious cycle that precludes successful applications of existing phosphor materials. The pcLED lamp for general illumination application requires high optical energy flux (e.g., higher than 1 Watt/mm2) which causes additional heating by a Stokes shift generated inside the phosphor crystals. Successful development of light emitting devices incorporating both phosphors and a light source, such as pcLED lamps for general illumination, therefore requires the development of phosphors that can operate highly efficiently at temperatures of 80° C.-150° C. The risk is that it is difficult both to achieve 90% quantum yield at room temperature and to have high thermal stability at 80° C.-150° C. The thermal stability of a phosphor's luminescence is an intrinsic property of the phosphor which is determined by both the composition and the structure of the crystalline material.
The use of carbon in phosphor preparations has previously been considered a source of quenching, rather than enhancing, the luminescence of a phosphor. For example, residual carbon that remains after phosphor preparation processes utilizing carbon can hinder the emission intensity of the phosphor. Further, the dark color of carbide materials naturally predisposes it to absorbing, rather than reflecting, light.
Carbidonitride phosphors are comprised of carbon, nitrogen, silicon, aluminum and/or other metals in the host crystal and one or more metal dopants as a luminescent activator. This class of phosphors recently emerged as a color converter capable of converting near UV (nUV) or blue light to green, orange and red light. The host crystal of carbidonitride phosphors is comprised of —N—Si—C— networks in which the strongly covalent bonds of Si—C and Si—N serve as the main structural components. Generically, the network structure formed by Si—C bonds has a strong absorption in the entire visible light spectral region, and therefore has been previously considered not suitable for use in host materials for high efficiency phosphors. For example, in certain nitride-silicon-carbide phosphors in which Ce3+ is the dopant, the electronic interaction between Ce3+ and the (—N—Si—C—) networks results in a strong absorption in 400-500 nm wavelengths, making the phosphor less reflective in that particular spectral region of visible light. This effect is detrimental to achieving a phosphor having a high emission efficiency.
It has now been discovered that in certain carbidonitride phosphor formulations, carbide actually enhances, rather than quenches, the luminescence of a phosphor, in particular at relatively high temperatures (e.g. 200° C.-400° C.). The present invention demonstrates that in certain carbidonitride phosphor formulations, the absorption in the visible light spectral region actually decreases as the amount of carbide increases. These carbide-containing phosphors have excellent thermal stability of emission and high emission efficiency.